Question : Cold Idle Issues

[HJR1]

Hey everyone, I finally finished buttoning up the install of my replacement engine and started it up for the first time on Thursday. I'm pleased to report that this engine sounds like a good one, runs smooth with no noise. I do have a few issues though so I have some questions, but I'll start with this one first:

When cold this truck idles/runs like a carbed engine does without a choke. It'll start up quick enough, but I have to nurse the throttle to keep it running or it'll die out. Not necessarily because of lack of idle either, if I hold it at a quick idle it'll still die out, I have to nurse the throttle up and back off. If I continue this until the engine reaches operating temperature it will then idle fine on it's own.

I found the vacuum line that feeds the 4wd and heater control is pretty wasted and leaking so I disconnected it and capped of the port for the time being. That helped a little but not much, all other vacuum lines appear to be okay and hooked up correctly.

Where do I start on this? It's a '87 S15 with the 2.8L V6. 5 speed, timing set at 10 degrees.

TIA!

[86FieroSEv6]

Is the throttle body base gasket leaking? Is the EGR valve leaking due to pintle carbon buildup? Do you have good suction on the PCV valve. Is the eng. temp. sensor input okay? What about fuel pressure and volume? I assume you disconnected the EST when you set the timing.

[bob442]

TPS or o2 would be my first guess, if you swapped the motor try changing that from the old one.
But, you say it runs fine till it warms up, i'm almost certian its the o2 then. it would be in close loop, they dont read properly till there warmed up. i guess make sure its even plugged in too.
if you want to test with propange for intake leaks you could do that aswell, just put a torch end on a hand led tank and move it slowly around the intake, you'll notice it run different when it sucks it in.

[HJR1]

Is the throttle body base gasket leaking? Is the EGR valve leaking due to pintle carbon buildup? Do you have good suction on the PCV valve. Is the eng. temp. sensor input okay? What about fuel pressure and volume? I assume you disconnected the EST when you set the timing.

1 No.

2 No.

3 Yes.

4 I do not know, can this be tested or is it a replace to see item?

5 Haven't checked this yet. Would weak pressure and/or volume affect cold running, but be okay when warmed up?

6 Yes.

Thank you for your help!

[HJR1]

TPS or o2 would be my first guess, if you swapped the motor try changing that from the old one.
But, you say it runs fine till it warms up, i'm almost certian its the o2 then. it would be in close loop, they dont read properly till there warmed up. i guess make sure its even plugged in too.
if you want to test with propange for intake leaks you could do that aswell, just put a torch end on a hand led tank and move it slowly around the intake, you'll notice it run different when it sucks it in.

Well, it's actually reverse of that, it idles/runs poorly until warmed up. I was thinking about replacing the o2 sensor anyway, it looks pretty old.

Thanks for the ideas!

[86FieroSEv6]

Sure, cold engines require more enrichment so early fuel supply problems will show up in the cold operation regime first.

[bob442]

TPS or o2 would be my first guess, if you swapped the motor try changing that from the old one.
But, you say it runs fine till it warms up, i'm almost certian its the o2 then. it would be in close loop, they dont read properly till there warmed up. i guess make sure its even plugged in too.
if you want to test with propange for intake leaks you could do that aswell, just put a torch end on a hand led tank and move it slowly around the intake, you'll notice it run different when it sucks it in.

i got that mixed upit it should be:
But, you say it runs fine when it warms up, i'm almost certian its the o2 then.

sorry, i should proof read my stuff more lol, go for the o2 sensor, and test the water tempature sensor too. do you know how to test them?

[HJR1]

Sure, cold engines require more enrichment so early fuel supply problems will show up in the cold operation regime first.

Gotcha, that makes since, I'll check that too.

i got that mixed upit it should be:
But, you say it runs fine when it warms up, i'm almost certian its the o2 then.

sorry, i should proof read my stuff more lol, go for the o2 sensor, and test the water tempature sensor too. do you know how to test them?

No, do you have the procedures for testing these? And is there two temp sensors? Looks like that's what I'm seeing under there.

Thanks again!

[bob442]

they usually dont fail, but you need a multi meter and a pot of boiling water, place the end of the senser in the hot water with the multi meter leads touching the ends, you'll have to put it on a OHM resistance, which one it is i dont remember, its going to be which ever one gives a reading when the sensor is cold, so anyways, place the sensor in the boiling or very hot water, and that reading should 'gradually' change. water tempature sensors take a second reading of the tempature of the motor, they too also play a roll in how lean or rich the fuel mixture is. one temp sensor maybe for the electric fan or check engine light. is there one with 2 wires that go to it? if so, check that one first. becarful you dont break the teminal on the sensor as they are worth 40-90$

On my ford truck, i had 2 problems, really bad surging idle on cold start and a less aggressive surge on warm idle, the cold surge when away by kicking the throttle ( i know it sounds funny because its FI) it ended up being a bad connection to the o2 sensor, the wire that heats up the o2, and the water tempature sensor for the hot idle. the reading didnt go that high (or low) when it was in hot water. 20 year old truck with almost 400km.

Check your o2's first, along with all the connections, makesure the wire isnt melted from a manifold or has green rot. I also fine o2's easier to remove when the manifold is hot, and using the proper sized wrench to break them loose.

[bszopi]

Or you could just go off the table listed here -> http://60degreev6.com/content/Detail...nsor_.28CTS.29

Figure out what your ambient temp is when measuring resistance and compare it to the values listed in the table.

[bob442]

Or you could just go off the table listed here -> http://60degreev6.com/content/Detail...nsor_.28CTS.29

Figure out what your ambient temp is when measuring resistance and compare it to the values listed in the table.

see, i didnt know about that... :P

Did anyone mention the idle air solinoid? if this truck even has one? see if it looks like its moved recently and that the passage way is clean.

[86FieroSEv6]

During cold start and warm up, the vehicle is in open loop . . . that is, NOT using O2 sensor input. Once a certain temperature is reached and the O2 sways from center then the vehicle will go into closed loop. What would be interesting to know, is which way the integrator and block learn values are moving in closed loop to compensate for the problem. If they are below 128 by a fair margin, then the vehicle is rich and the system is trying to lean it out. If they are significantly above 128, then the system is lean and enrichment is being commanded. Knowing this information will help to infer the nature of the open loop control problem.